Few things about this guaranteed update thing:
1. Guaranteed update is fine, but it won't affect non-Nexus devices, which are by far the majority of devices out on the market.
2. Guaranteed update doesn't mean a fast update. Some devices are still waiting for 5.1.1 update despite the Android One smartphones getting it back in in February...
3. I don't know why Google feel they need to announce this. I thought the assumption was that Nexus devices would get updates and was supposed to get them faster than OEM phones. If they're going to come out and announce this thing, then they better back it up.
5.1.1 release was abysmal, with 3rd party OEM devices getting the update faster than Nexus devices. That's just not acceptable.